Multi-cationic aluminate spinels

ABSTRACT

The present invention is directed to both stoichiometric and sub-stoichiometric high entropy aluminate spinels as a novel high entropy oxide (HEO) crystal phase. Previously reported HEOs are overwhelmingly stoichiometric structures containing a single cationic site and are stabilized solely by intermixing increasing numbers of cations. According to an aspect of the invention, sub-stoichiometric spinels, containing various mixtures of divalent metal cations and cationic vacancies in nominally equimolar concentration, provide entropic stabilization similarly to cations in stoichiometric spinels. The chromatic, structural, and chemical properties of these complex spinels are highly tunable via incorporation of cationic vacancies and multiple divalent metals, enabling their application as unique pigments, catalysts, and thermal coatings.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.63/337,232, filed May 2, 2022, which is incorporated herein byreference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Contract No.DE-NA0003525 awarded by the U.S. Department of Energy/National NuclearSecurity Administration. The Government has certain rights in theinvention.

FIELD OF THE INVENTION

The present invention relates to high entropy oxides and, in particular,to multi-cationic aluminate spinels.

BACKGROUND OF THE INVENTION

High entropy oxides (HEOs) are a relatively new class of materialscontaining multiple cations (typically four or more) consolidated into asingle-phase crystal structure. These structures are stabilized throughconfigurational entropy (Se), which increases with the number of uniquearrangements of cations and anions within the crystal structureaccording to Equation 1,

S _(c) =−k ₈[(Σ_(i=1) ^(N) x _(i) In(x _(i)))_(cation)+(Σ_(j=1) ^(M) y_(j) In(y_(j)))_(anion)]  (1)

where x_(i) and y_(j) are mole fractions of each cationic and anionicspecie, respectively, and N and M are the total number of uniquecationic and anionic species, respectively. See C. M. Rost et al., Nat.Commun. 6, 8485 (2015). For a given N and M, S_(c) is maximized whenspecies are in equimolar concentration. The tendency of constituentcations to adopt dissimilar secondary oxide phases is overcome whenincreased entropy outweighs enthalpic energy penalties associated withmixing, according to the Gibbs equation (ΔG=ΔH−TΔS). High entropy oxideswhich exhibit this thermodynamic phenomenon are further distinguished asentropy—stabilized oxides. Previous studies report HEOs adopting avariety of different crystal structures, including rock salt, fluorite,spinel, and perovskite, which have demonstrated advantageous ionicconductivity, thermal conductivity, dielectric constants, and catalyticproperties. See C. M. Rost et al., Nat. Commun. 6, 8485 (2015); A.Sarkar,et al., Nat. Commun. 9, 3400 (2018); J. Gild et al., J. Eur.Ceram. Soc. 38, 3578 (2018); M. R. Chellali et al., Scr. Mater. 166, 58(2019); Z. Grzesik et al., J. Eur. Ceram. Soc. 40, 835 (2020); D. Wanget al., J. Mater. Chem. A 7, 24211 (2019); Y. Zhang et al., ACS Appl.Mater. Interfaces 12, 32548 (2020); R. Banerjee et al., ACS Sustain.Chem. Eng. 8, 17022 (2020); A. Sarkar et al., J. Eur. Ceram. Soc. 38,2318 (2018); D. Bérardan et al., J. Mater. Chem. A 4, 9536 (2016); D.Bérardan et al., Phys. Status Solidi Rapid Res. Lett. 10, 328 (2016); H.Chen et al., J. Mater. Chem. A 6, 11129 (2018); and C. Riley et al., ACSAppl. Mater. Interfaces 13, 8120 (2021). These oxides are stabilizedsolely by increasing the number of cationic elements mixed together andincreasing the temperature until a phase pure structure formed. Despiterecent development of complex perovskite and transition metal spinelHEOs, many of these studies focused on stoichiometric crystal structureshaving a single equivalent cationic site, an approach which ultimatelylimits the number of parameters available to design unique complexoxides. See Z. Grzesik et al., J. Eur. Ceram. Soc. 40, 835 (2020); J.Dabrowa et al., Mater. Lett. 216, 32 (2018); and J. Dabrowa et al.,Materials (Basel) 14, 5264 (2021).

SUMMARY OF THE INVENTION

The present invention is directed to a multi-cationic aluminate spinel,comprising at least three different divalent metal cations in nominallyequimolar concentrations in an aluminate spinel crystal structure. Themulti-cationic aluminate spinel can comprise a stoichiometric amount ofat least four different divalent metal cations in nominally equimolarconcentrations. Alternatively, the multi-cationic aluminate spinel cancomprise sub-stoichiometric amounts of the at least three differentdivalent metal cations in nominally equimolar concentrations to aconcentration unoccupied vacancy sites. The divalent metal cations canbe selected from a group consisting of alkaline-earth metals, such asmagnesium and calcium, and transition metals, such as cobalt, copper,manganese, nickel, chromium, iron, and zinc, which are known to producemetal aluminate spinels.

The invention is further directed to a method of dry reforming ofmethane, comprising providing the Ni-based multi-cationic aluminatespinel catalyst, and exposing a stream comprising methane and carbondioxide to the Ni-based multi-cationic aluminate spinel catalyst at anelevated temperature, thereby catalytically reacting the methane andcarbon dioxide to produce a synthesis gas. Although the reaction canoccur at temperatures as low as 400° C., the reaction temperature ispreferably greater than 600° C. to achieve significant conversion ofmethane and carbon dioxide. The exposed, spent Ni-based multi-cationicaluminate spinel catalyst can be regenerated under oxidizing conditions.

Complex aluminate spinels stabilized with cation vacancies are a newhigh entropy oxide material system. The unique ability of aluminatespinels to accommodate large deviations below stoichiometry enablesvacancy-contributed configurational entropy. Cationic and anionicvacancies, as well as cation inversion, contribute to configurationalentropy and aid the stabilization of phase pure complex spinel oxides.Entropy effects, as well as sol-gel synthesis, significantly lowers theprocessing temperature required to form phase pure spinels compared toconventional oxides produced through solid-state reactions.Incorporation of high cation vacancy concentrations and multipledivalent metals yields highly tunable structural, chromatic, andchemical properties. These properties enhance the potential of highentropy and sub-stoichiometric spinels as unique pigments, photoabsorbents, refractories, and catalysts. Finally, the concept ofvacancy-stabilized high entropy oxides can be applied to othernon-stoichiometric complex oxides with crystal structures havingmultiple cationic lattice sites, including other spinel, perovskite, andpyrochlore material systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description will refer to the following drawings, whereinlike elements are referred to by like numbers.

FIG. 1 is a schematic illustration of an aluminate spinel, a highentropy aluminate spinel comprising five different cations, and avacancy-stabilized high entropy aluminate spinel comprising bothcationic and anionic vacancies.

FIG. 2A shows x-ray diffraction (XRD) patterns of stoichiometric simplespinels. FIG. 2B shows XRD patterns of alumina and magnesium aluminateswith varying Mg stoichiometry all calcined at 800° C.

FIGS. 3A and 3B show XRD patterns of stoichiometric spinels containingvarying numbers of divalent metal cations. FIGS. 3C-3F show XRD patternsof sub-stoichiometric spinels containing cationic vacancies. All samplesshown were calcined at 800° C.

FIG. 4 shows a scanning electron microscopy (SEM) image (left) andelemental mapping (right) of the (CoCuMgMnNi)₁ sample showing amorphologically porous, compositionally homogeneous microstructure.Scale marker is 50 μm.

FIG. 5A is a transmission electron microscopy (TEM) image of the(CoMgNi)_(0.75) sample showing nanostructure morphology at lowermagnification. FIG. 5B is a TEM image at higher magnification showinginterplanar spacing of confirmed spinel planes.

FIG. 6A shows reflectance spectra of nickel aluminate spinels withvarying Ni²⁺:Al³⁺ ratio. FIG. 6B shows reflectance spectra of nickelaluminate spinels with the addition of other divalent metal cations.

FIG. 7 is a graph of hydrogen temperature programmed reduction (H₂-TPR)profiles of select Ni-containing spinel samples.

FIG. 8A is a schematic illustration of a dry reforming of methane (DRM)reaction and oxidative regeneration of a convention nickel catalyst.FIG. 8B is a schematic illustration of a DRM reaction and oxidativeregeneration of a multi-cationic aluminate spinel catalyst.

FIGS. 9A and 9B are graphs showing activation profiles of variouscatalysts for CH₄ and CO₂ conversion, respectively, according to the DRMreaction. In the graphs, the samples are labeled as X with the formulaXAl₂O₄. For example, the sample labeled Ni₁ is an oxide with compositionNiAl₂O₄. On stream conditions were temperature=775° C.; total flowrate=24 ml/min; gas composition=20% methane, 20% carbon dioxide, and 60%nitrogen; and catalyst mass=20 mg.

FIG. 10A shows an SEM image (left) and FIG. 10B shows an elementalmapping (right) of a sub-stoichiometric (CoMgNi)_(0.75) sample after DRMreaction. FIG. 10C shows an SEM image (left) and FIG. 10D shows anelemental mapping (right) of a sub-stoichiometric (CoNi)_(0.67) sampleafter DRM reaction.

FIG. 11 is a thermogravimetric analysis (TGA) of various spent spinelcatalysts after DRM. The TGA mass change indicates coke evolution.

FIG. 12A shows XRD patterns of a NiA₁₂O₄ spinel before and after DRMreaction and after exposure of the spent spinel to oxidizing conditions.FIG. 12B shows XRD patterns of a (CoMgNi)_(0.75) aluminate spinel beforeand after DRM reaction and after exposure of the spent spinel tooxidizing conditions. The multi-cationic aluminate spinel is fullyregenerable due to entropic stabilization.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to both stoichiometric andsub-stoichiometric high entropy aluminate spinels as a novel highentropy oxide (HEO) crystal phase. An aspect of the invention isdirected to the stabilization of HEOs with cationic vacancies. From aconfigurational entropy perspective, site vacancies constitute a uniquechemical specie that increases the number of unique atomicconfigurations within a crystal lattice in the same manner as a cation.See O. F. Dippo and K. S. Vecchio, Scr. Mater. 201, 113974 (2021); andA. Sarkar et al., Scr. Mater. 187, 43 (2020). Unfortunately, most metaloxide crystal structures do not support large vacancy concentrations,preventing their systematic study. Aliovalent cation dopantincorporation and oxygen partial pressure change are shown to inducevacancies in both compositionally simple and high entropy oxides. See Y.Zhang et al., ACS Appl. Mater. Interfaces 12, 32548 (2020); D. Bérardanet al., J. Mater. Chem. A 4, 9536 (2016); C. Riley et al., ACS Appl.Mater. Interfaces 13, 8120 (2021); B. Bulfin et al., Phys. Chem. Chem.Phys. 18, 23147 (2016); and R. Djenadic et al., Mater. Res. Lett. 5, 102(2016). However, vacancy concentrations induced through these methodsare typically too low to significantly increase configurational entropy.See F. Yuan et al., J. Phys. Chem. C 119, 13153 (2015); and A. M.Ferrari et al., J. Chem. Phys. 127, 174711 (2007). Sincecrystallographic defects are known to enhance many of the usefulproperties of metal oxides (ionic conductivity, energy storage, andcatalytic activity, for instance), a systematic study ofdefect-stabilized HEOs would be valuable. See C. Riley et al., ACS Appl.Mater. Interfaces 13, 8120 (2021); M. Moździerz et al., Acta Mater. 208,116735 (2021); and X. Liu et al., Small 18, e2200524 (2022). Further,cationic vacancies are accompanied by charge-compensating anionicvacancies, enabling a configurational entropy contribution from theanionic sublattice, which is typically assumed to be zero in metaloxides. See C.-J. Ting and H.-Y. Lu, J. Am. Ceram. Soc. 82, 841 (1999);and N. Osenciat et al., J. Am. Ceram. Soc. 102, 6156 (2019). Highentropy carbides, silicides, borides, and sulfides offer an alternativeto oxygen anions in compositionally complex materials. See E. Castle etal., Sci. Rep. 8, 8609 (2018); P. Sarker et al., Nat. Commun. 9, 4980(2018); J. Gild et al., J. Materiomics 5, 337 (2019); Y. Qin et al., J.Adv. Ceram. 8, 148 (2019); J. Gild et al., Sci. Rep. 6, 37946 (2016); Y.Zhang et al., J. Eur. Ceram. Soc. 39, 3920 (2019); Y. Zhang et al., Scr.Mater. 164, 135 (2019); R. Z. Zhanget al., Inorg. Chem. 57, 13027(2018); C. R. McCormick and R. E. Schaak, J. Am. Chem. Soc. 143, 1017(2021); and M. Cui et al., Adv. Energy Mater. 11, 2002887 (2020).Several studies of high entropy carbides demonstrate significant anionicvacancy concentrations shown to stabilize the carbide structure, whichsuggests the same principle may stabilize other material systems, likemetal oxides. See C. Peng et al., J. Mater. Sci. Technol. 51, 161(2020); and Y. He et al., J. Mater. Sci. 55, 6754 (2020). However, amethod for supporting high vacancy concentrations is lacking inpreviously studied HEOs.

More particularly, the present invention is directed to HEOs having analuminate spinel crystal structure. FIG. 1 is a simplified illustrationof spinel crystal structures, including an aluminate spinel (MAl₂O₄), ahigh entropy aluminate spinel comprising five different divalent metalcations (M_(i)), and a vacancy-stabilized aluminate spinel comprisingvacancies occupying both cationic (V_(c)) and anionic (V_(a)) sites.These spinels are AB₂O₄ type oxides in which A-site cations havetetrahedral coordination and B-site cations have octahedralcoordination. Stoichiometric aluminate spinels have MAl₂O₄ composition,where M is a divalent metal cation. However, previous studies observedthat aluminates with divalent cation concentrations significantly belowstoichiometric values retained a spinel crystal structure byaccommodating cationic site vacancies and charge-compensating oxygenvacancies. See Y. Li et al., Ceram. Int. 44, 15104 (2018); S. Zhang etal., Appl. Catal. B 291, 120074 (2021); and M. Gil-Calvo et al., Appl.Catal. B 209, 128 (2017). In fact, y-alumina, a common Al₂O₃ phasehaving no divalent cations, adopts a defected spinel structurecontaining site vacancies. See R. Prins, J. Catal. 392, 336 (2020); andM. Lee et al., Chem. Phys. Left. 265, 673 (1997). Thus, these materialsare uniquely suited for increased vacancy-contributed configurationalentropy. The presence of two cationic sublattices also enables siteinversion, a unique structural feature that can increase configurationalentropy. See H. St. C. O'Neill et al, Phys. Chem. Miner. 18, 302 (1991);and J. Yan et al, Ceram. Int. 45, 14073 (2019). Inversion occurs whendivalent cations occupy octahedral sites instead of tetrahedral, andtrivalent cations occupy tetrahedral sites instead of octahedral. Theinversion parameter describes the extent to which this phenomenon occursand is given as x in the formula (A_(1-x)B_(x))(B_(2-x)A_(x))O₄ forstoichiometric spinels, where x =0 in normal spinels and x=1 in fullyinverse spinels. Although Al³⁺ ions prefer octahedral coordination, theenergetics of divalent cations often dictate site occupation of bothcations within a spinel structure. See D. McClure, J. Phys. Chem. Solids3, 311 (1957); and B. Lavina et al., Phys. Chem. Miner. 29, 10 (2002).Ni²⁺ and Cu²⁺ energetically favor octahedral site occupation and forminverse aluminate spinels, but CuAl₂O₄ with highly disordered normalspinel structure has also been observed. See H. St. C. O'Neill et al.,Eur. J. Mineral. 17, 581 (2005). On the other hand, Mn²⁺ and Co²⁺ showlittle energetic preference, and Mg²⁺ favor tetrahedral sites. See D.McClure, J. Phys. Chem. Solids 3, 311 (1957). Accounting for sitevacancies and cation inversion, a formula for configurational entropyfor non-stoichiometric spinels is given as Equation 2. Non-zeroconfigurational entropy contributions from two cationic and an anionicsublattice clearly offers a unique opportunity for entropicstabilization within complex spinels, which is inaccessible in manypreviously studied HEO materials.

$\begin{matrix}{S_{c} = {- {k_{B}\left\lbrack {\left( {{\Sigma}_{i = 1}^{N}x_{i}{\ln\left( x_{i} \right)}} \right)_{\underset{cation}{A - {{si}te}}} + \left( {{\Sigma}_{i = 1}^{N}x_{i}{\ln\left( x_{i} \right)}} \right)_{\underset{cation}{B - {site}}} + \left( {{\Sigma}_{j = 1}^{M}y_{i}{\ln\left( y_{j} \right)}} \right)_{anion}} \right\rbrack}}} & (2)\end{matrix}$

As an example of the invention, aluminate spinels were produced withincreasing numbers of divalent cationic species, including Co, Cu, Mg,Mn, and Ni ions, and cationic vacancies in varying combinations.Cationic vacancies were induced by using metal precursors with M²⁺:Al³⁺ratios <0.5 in a facile sol-gel synthesis. Stoichiometric andsub-stoichiometric samples containing one to five cationic species weresystematically heated from 800-950 ° C. and measured for phase purity.Sample XRD patterns were compared to those generated from modeled spinelcrystal structures to determine average cationic site occupation andvacancy concentration, similar to methods reported in previousliterature. See J. Yan et al, Ceram. Int. 45, 14073 (2019); and R. F.Cooley and J. S. Reed, J. Am. Ceram. Soc. 55, 395 (1972). Due to therelatively high energetic barrier for NiAl₂O₄ formation, the examplesdescribed below primarily relate to spinels containing Ni. Samples wereevaluated for chromatic, chemical, thermal, and catalytic properties,which are relevant to the primary applications of aluminate spinels asaesthetic pigments, catalysts, and refractory materials. See M. Gaudonet al., Ceram. Int. 40, 5201 (2014); G. Buvaneswari et al., Dyes Pigm.123, 413 (2015); S. Zhang et al., Appl. Catal. B 291, 120074 (2021); andH. Tang et al., Materials(Basel) 14, 3050 (2021). These properties arehighly tunable through the inclusion of both multiple divalent cationsand cationic vacancies, as compared to simple spinels.

Synthesis of Aluminate Spinels

Spinel samples were synthesized through a sol-gel synthesis previouslyreported, with several adjustments. See C. Riley et al., ACS Appl.Mater. Interfaces 13, 8120 (2021); C. Riley et al., ChemCatChem 11, 1526(2019); and C. Riley et al., Appl. Catal. B 264, 118547 (2020). First, 5g of polyvinylpyrrolidone was dissolved in 100 ml of DI water undervigorous stirring. Metal nitrate precursors were then added to thepolymer solution and stirred for 1 hour. A total of 7 mmol of divalentmetal nitrates were added to make stoichiometric spinels or less than 7mmol for sub-stoichiometric spinels. Divalent metal precursors includedcobalt (II) nitrate hexahydrate, copper (II) nitrate hemipentahydrate,magnesium (II) nitrate, manganese (II) nitrate tetrahydrate, and nickel(II) nitrate hexahydrate. 14 mmol of aluminum (III) nitrate nonahydratewas added. The resulting solutions were heated at 110° C. overnight toevaporate water and form hard gels. These gels were crushed to a coarsepowder and placed in the center of a vented box furnace preheated at800° C. for 2 hours to calcine within a fume hood. One set of sampleswas subjected to the same calcination parameters but with ramp rates of5 and 25° C/min to evaluate this effect on phase purity. Calcinedsamples containing secondary oxide phases were further heated attemperatures between 850 and 950° C. for 2 hours followed by quenchingin ambient air to demonstrate the effect of temperature on phase purespinel formation. For simplicity, spinel samples are labeled hereinaccording to the constituent divalent metals and their nominalconcentration. Parentheses are used to designate equimolarconcentrations of the enclosed elements, which are listed inalphabetical order. For instance, stoichiometric spinel containing onlynickel is labeled Ni₁, and stoichiometric spinel containing both nickeland magnesium is labeled (NiMg)₁. Sub-stoichiometric spinels are labeledwith A-site cation stoichiometry values less than 1, such as for thesample having equimolar concentrations of nickel, magnesium, andcationic vacancies labeled as (NiMg)_(0.67), wherein 1/3 of A-sites areassumed to be vacant. Where appropriate, samples are also labeled withthe highest heat treatment temperature used to compare the effect ofaging on single phase formation. Otherwise, samples labeled without atemperature underwent 800° C. calcination as the sole heat treatment.

Characterization of Aluminate Spinels

Initial characterization results yielded compositional and structuralinformation of spinel samples containing a single divalent cation innominal stoichiometric ratio to aluminum. XRD patterns of these samplesare shown in FIG. 2A. Data are plotted as the square root of intensityto enhance the visibility of any peaks corresponding to lowconcentrations of secondary oxide phases. As with previous HEO studies,the cationic constituents added here demonstrate varying tendency toform secondary oxide phases dissimilar to that of the spinel structure.See C. M. Rost et al., Nat. Commun. 6, 8485 (2015). Co₁ and Mg₁ samplescalcined at 800° C. adopt phase pure CoAl₂O₄ and MgAl₂O₄ spinelstructures, respectively, while Cu₁ and Ni₁ samples contained segregatedoxide phases. Cu²⁺ and Ni²⁺ ions have a higher preference for octahedralsite occupancy than do Co²⁺ and Mg²⁺. See D. McClure, J. Phys. Chem.Solids 3, 311 (1957). Enthalpies of formation (ΔH_(f)) of CuAl₂O₄ andNiAl₂O₄ are also notably higher than for CoAl₂O₄ and MgAl₂O₄, whichaccounts for segregation of CuO and NiO phases under initial synthesisconditions. See A. Navrotsky and O. Kleppa, J. Inorg. Nucl. Chem. 30,479 (1968). Mn₁ shows poor crystallinity, as evidenced by broad peaks oflow intensity. The low intensity of the Co₁ sample is likely a result offluorescence from cobalt, which raises the background signal. Table Ilists electron probe microanalysis (EPMA) compositional data, showingthat the actual concentrations of divalent cations were near nominalvalues. Since Mg showed a tendency to form a phase-pure spinel, a seriesof samples with varying Mg concentration were produced to demonstratethe ability to form sub-stoichiometric compounds. As shown in FIG. 2B,the pure alumina sample synthesized without divalent metal addition hasy-Al₂O₃ phase in which Al³⁺ ions are known to favor octahedral sites.See M. Lee et al., Chem. Phys. Lett. 265, 673 (1997). Samples with Mgstoichiometry from 0.2-1 all adopted a phase pure spinel structure. Thisresult verifies that alum inate spinels accommodate large deviationsfrom stoichiometry and thus large cation vacancy concentrations. Thesefindings are corroborated by previous studies. See S. Zhang et al.,Appl. Catal. B 291, 120074 (2021); and M. Gil-Calvo et al., Appl. Catal.B 209, 128 (2017).

TABLE I Physical characterization of stoichiometric spinels with asingle divalent cation. Calcination Lattice Average M²⁺ compositiontemperature parameter crystallite size Surface area Sample (x inM_(x)Al₂O₄) (° C.) (Å)^(a) (nm)^(a) (m²/g) Co₁ 0.99 800 8.095 19.4 52Cu₁ 1.03 950 8.091 20.6 8 Mg₁ 0.90 800 8.078 5.8 95 Mn₁ 1.06 9508.084^(b) 7.4^(b) 29 Ni₁ 0.87 950 8.060 9.4 58 ^(a)Lattice parametersand average crystallite sizes listed correspond to MAl₂O₄ phase^(b)Values are given for the predominant MAl₂O₄ phase

The effects of heat treatment and the number of divalent cationicspecies (N) on entropic stabilization of single-phase stoichiometricspinels was evaluated. Cu₁, Mn₁, Ni₁ samples previously calcined at 800°C. all showed secondary oxide phases. These samples were next calcinedat 900 and 950° C. for 2 hours, followed by quenching in ambient air.XRD patterns indicated that calcination at 950° C. followed by quenchingwas sufficient to create phase pure NiAl₂O₄ and CuAl₂o₄ structures.Raman spectra of the Ni₁-800° C. and Ni₁-950° C. showed vibrationalfeatures corresponding to the NiAl₂O₄ phase. See Y. Gao et al., Catal.Lett. 150, 3617 (2020). However, a peak near 1100 cm⁻¹ corresponding toa multi-phonon scattering mode of NiO was present in the sample calcinedat 800° C., but was absent in the sample calcined at 950° C. See N.Mironova-Ulmane et al., J. Phys. Conf. Ser. 93, 012039 (2007). The peaksof quenched Cu₁ were asymmetric, indicating a distribution of unit cellsizes skewed toward larger dimensions, which is likely caused bynon-equilibrium mixed cation site occupation retained through rapidsample cooling. Secondary oxide phases segregated when these sampleswere allowed to cool slowly within the furnace, as opposed to quenched.Previous studies observed this behavior within entropically stabilizedmaterials and was demonstrated through XRD patterns of the Ni₁ sample.Even after aging at 950° C., Mn₁ samples contained multiple phases,including two distinct MnAl₂O₄ phases and Mn₃O₄. According to crystalfield theory, Mn²⁺ ions can occupy tetrahedral and octahedral sites withno energetic preference but with different Mn—O bond lengths, whichexplains why two unique spinel crystal structures with distinct peaklocations and lattice parameters coexist within the Mn₁ sample. See D.McClure, J. Phys. Chem. Solids 3, 311 (1957). Lattice parameter andaverage crystallite size are listed for the predominant MnAl₂O₄ phase inTable I. While aging at elevated temperature was effective in producingphase pure spinels, samples were prone to sintering. Ni₁ calcined at800° C. had a surface area of 117 m²/g, which dropped to 52 m²/g afterheating at 950° C. Such effects hamper performance of spinels insurface-mediated applications, like catalysis, which incentivizesproduction of spinels at lower temperatures. This effect has beenachieved in previous HEO studies through mixing of additional cations.See C. M. Rost et al., Nat. Commun. 6, 8485 (2015).

To promote configurational entropy through cationic mixing, a set ofnickel-containing spinels was synthesized with multiple divalent metals.These stoichiometric samples were all calcined at 800° C. and includedNi₁, (MgNi)₁, (CoMgNi)₁, (CoCuMgNi)₁, and (CoCuMgMnNi)₁, with thecorresponding number of cationic species being N=1 through N=5. XRDpatterns in FIGS. 3A and 3B show the diminishing presence of NiO withincreasing N. Rietveld refinements confirmed the presence of segregatedNiO in (CoMgNi)₁, but the absence of this phase in (CoCuMgNi)₁. Thepattern of (CoCuMgMnNi)₁ additionally showed no phases other thanaluminate spinel. SEM-EDS mapping of this sample indicates a homogeneousdistribution of all constituent elements, as shown in FIG. 4 . EPMAanalysis further confirmed that actual compositions were near equimolarvalues, as shown in Table II. The effect of calcination parameters wasevaluated by comparing the structure of (CoCuMgNi)₁ samples all heatedto 800° C. but at varying ramp rates (5° C/min, 25° C./min, and withplacement in a preheated furnace). All three structures were phase purewith no significant structural differences observed via XRD patterns,suggesting that a phase pure high entropy spinel is indeedthermodynamically favored at this calcination temperature. By raisingentropy via increased temperature and/or number of cationicconstituents, enthalpic penalties for nickel aluminate formation wereovercome, resulting in stabilization of phase pure spinel HEOs. Further,the sol-gel synthesis enabled compositionally homogeneous oxideformation through aqueous precursor mixing, negating high temperaturesolid-state mixing protocols common in HEO studies.

TABLE II Physical characterization of stoichiometric spinels withmultiple divalent cations. Lattice Average Divalent metal stoichiometryparameter crystallite Sample Co Cu Mg Mn Ni (Å) size (nm) Ni₁ — — — —0.87 8.060 9.4 (MgNi)₁ — — — — 8.070 14.1 (CoMgNi)₁ 0.33 — 0.39 — 0.338.069 8.5 (CoCuMgNi)₁ 0.26 0.26 0.23 — 0.26 8.073 11.2 (CoCuMgMnNi)₁0.20 0.18 0.28 0.21 0.20 8.089 12.0

Vacancy-Stabilized Aluminate Spinels

Next, sub-stoichiometric spinels were synthesized with 800° C.calcination and characterized. Ni—containing spinels were again studied,and cationic elements were added in the same order as in the previousset of samples. However, the ratio of total divalent cations to Al³⁺ was<0.5, and the concentration of each divalent element was kept equimolarto the concentration of unoccupied divalent cation sites, assuming thesedivalent species occupy A-sites only. The resulting samples wereNi_(0.5), (MgNi)_(0.67), (CoMgNi)_(0.75), and (CoCuMgNi)_(0.8).Compositions, lattice parameters, and average crystallite sizes of theseoxides are listed in Table III. Total divalent metal cationconcentrations are clearly below stoichiometric values, due to vacancyinclusion. Divalent species and vacancies are in nominally equimolarconcentrations. Although entropy is maximized when the species are inequimolar concentration, according to Equation 1, the results in TablesII and III indicate that entropic stabilization can be achieved withmolar concentrations that deviate from strictly equimolar (e.g., by ±40%of the equimolar concentration). Indeed, for catalyst applications, itmay be beneficial to have catalytically active species present in higherconcentrations. At any rate, elemental concentrations should not be sodissimilar that they can no longer be consolidated into a single crystalphase through thermal treatments.

FIGS. 3C-3D show the corresponding XRD patterns. As with thestoichiometric spinels, segregated NiO phase is present in spinelshaving up to three cationic species. (CoMgNi)_(0.75) and(CoCuMgNi)_(0.8) samples having four and five cationic species, on theother hand, are phase pure. TEM imaging of the (CoMgNi)_(0.75) sampleconfirmed a spinel structure composed of agglomerated nanoparticles,shown in FIGS. 5A and 5B. Interplanar distances in the lattice fringesmeasured with ImageJ software were 4.6 and 2.4 Å, which can be indexedto the (111) and (311) planes, respectively, of the nickel and magnesiumaluminate spinels.

TABLE III Physical characterization of sub-stoichiometric spinels withmultiple divalent cations. Divalent specie stoichiometry CalcinationLattice Average M²⁺ temperature parameter crystallite Sample Co Cu Mg NiVacancy (° C.) (Å) size (nm) Ni_(0.5) — — — 0.49 0.51 950 8.033 4.2(MgNi)_(0.67) — — 0.42 0.28 0.30 900 8.061 6.7 (CoMgNi)_(0.75) 0.24 —0.32 0.25 0.19 800 8.082 8.5 (CoCuMgNi)_(0.8) 0.2 0.18 0.27 0.21 0.14800 8.076 13.9

In HEO spinels, divalent cations compete for limited available latticesites, and varying diffusion rates could limit the integration ofkinetically slower species, leading to segregated oxide phases. To ruleout the competing effects of other divalent cations, Ni_(0.25) and(MgNi)_(0.5) samples were evaluated via XRD, as shown in FIGS. 3E-3F,which had lower Mg concentrations and higher vacancy concentrations thanthose previously shown in an effort to increase Ni diffusion rates. SeeG. Yamaguchi et al., Bull. Chem. Soc. Jpn. 42, 2801 (1969). Despitethese factors, NiO segregation was evident in both Ni_(0.25) and(MgNi)_(0.5) samples, suggesting that Ni ions are not kineticallyhindered from incorporating into the spinel lattice under thesesynthesis conditions. Detection of the NiO phase within the Ni_(0.25)sample further excludes decreased nickel content as an explanation forthe absence of detectable NiO peaks in XRD patterns of phase pure(CoMgNi)_(0.75) and (CoCuMgNi)₁. However, segregation of NiO (even atlow Ni loadings) is rational in the context of entropic stabilization,in that a lower number of cationic species and deviation from equimolarcation concentrations does not provide enough entropy to overcome therelatively high enthalpic terms associated with nickel aluminate phaseformation. See A. Navrotsky and O. Kleppa, J. Inorg. Nucl. Chem. 30, 479(1968). The persistent finding that four cationic species are requiredto stabilize phase pure Ni—containing spinels at the same temperature isconsistent with the principles of entropic stabilization. Since thisbehavior is consistent between stoichiometric and sub-stoichiometricspinels, it can be concluded that cationic vacancies contribute toconfigurational entropy similarly to cations and can stabilize phasepure HEOs.

The structural features of select samples were further evaluated toestimate cationic and anionic site occupation. Measured XRD patternswere fit via Rietveld refinement and compared to simulated patterns withvarying cation compositions and site occupancies, as demonstrated inprevious literature. See J. Yan et al, Ceram. Int. 45, 14073 (2019); andT. Tangcharoen et al., J. Mol. Struct. 1182, 219 (2019). Becausediffraction peak intensity is a function of the size and electrondensity of atoms within a given crystallographic plane, relative peakintensities can be used to determine the distribution of cationsoccupying A and B sites. As exemplified by nickel aluminates, simulatedpatterns show variation in relative peak intensity, most notably between(111) and (220) peaks, when changing the relative site occupation ofNi²⁺ and Al³⁺ cations. Measured patterns of Mg₁ and Ni₁ samples werefirst evaluated, which are known to adopt normal and inverse spinelstructures, respectively. As expected, Rietveld refinement indicatesthat all Mg²⁺ ions occupy tetrahedral A-sites and all Ni²⁺ ions occupyoctahedral B-sites, while oxygen sites in these stoichiometricstructures are completely filled, as shown in Table IV. This findingcorroborates the entropic stabilization of NiAl₂O₄ observed in the heattreatment studies, as well as prior literature. See H. St. C. O'Neill etal, Phys. Chem. Miner. 18, 302 (1991). The presence of both Al²⁺ andNi²⁺ ions in B-sites creates non-negligible configuration entropy. Ni²⁺ions occupy B-sites exclusively in non-stoichiometric Ni_(0.75) andNi_(0.5) samples as well. Refinements of these patterns also indicatethe presence of cationic and anionic vacancies. The total occupation ofcationic and anionic sites decreases in spinels with low Ni²⁺:Al³⁺ratios and indicates that the non-stoichiometric structures are highlydefected. A decrease in spinel lattice parameter is also consistent withhigher vacancy concentrations. See Y. Li et al., Ceram. Int. 44, 15104(2018); and S. Zhang et al., Appl. Catal. B 291, 120074 (2021). Spinetscontaining multiple divalent cations are more difficult to model, andthese species were treated as a single element with electron densitythat was the average of all divalent cationic species. As shown in TableIV, Al³⁺ and M²⁺ cations occupy both tetrahedral and octahedral siteswithin (CoCuMgNi)₁ and (CoMgNi)_(0.75) samples. However, the degree ofinversion is lower in these HEO spinels, with a majority of M²⁺ ionsoccupying tetrahedral sites. This is expected with addition of Mg²⁺ andCo²⁺ ions, which have much higher energetic preference for tetrahedralsite occupation than Ni²⁺. See D. McClure, J. Phys. Chem. Solids 3, 311(1957). Inclusion of both divalent cations that prefer normal andinverse spinel structures creates mixed occupations on both A and Blattice sites. The complex structures that result from multiple cationicconstituent inclusion and with non-stoichiometric ratios enables allthree sub-lattices of aluminate spinels to contribute configurationalentropy, which is inherently unachievable in many HEO material systems.

TABLE IV Measured compositions and site occupancies modeled throughRietveld refinement of select spinel samples. Measured total Lattice M²⁺composition parameter A-site B-site O-site Sample (x in M_(x)Al₂O_(z))(Å) occupancy occupancy occupancy Mg₁ 0.90 8.078 Mg₁ Al₂ O₄ Ni₁ 0.998.056 Al₁ Al₁Ni₁ O₄ Ni_(0.75) 0.76 8.047 Al_(0.98) Al_(1.02)Ni_(0.76)O_(3.82) Ni_(0.5) 0.49 8.032 Al_(0.9) Al_(1.1)Ni_(0.56) O_(3.56)(CoCuMgNi)₁ 1.01 8.077 Al_(0.2)M_(0.69) Al_(1.8)M_(0.22) O_(3.35)(CoMgNi)_(0.75) 0.81 8.069 Al_(0.1)M_(0.52) Al_(1.9)M_(0.1) O_(3.6)

Optical Properties of Multi-Cationic Aluminate Spinels

The role of stoichiometry and compositional complexity on materialproperties relevant to aluminate spinels applications was evaluated.NiAl₂O₄, CoAl₂O₄, and CuAl₂O₄ spinels commonly serve as cyan, blue, andred-brown pigments in paints, dyes, inks, and other aesthetic coatings.See B. Serment etal., ACS Omega 5, 18651 (2020); L. Torkian and M.Daghighi, Adv. Powder Technol. 25, 739 (2014); and T. Tangcharoen etal., J. Adv. Ceram. 8, 352 (2019). The color of these metal oxides isknown to be influenced by structural parameters relevant to thesynthesis method used and transition metal constituents. See T.Tangcharoen et al., J. Adv. Ceram. 8, 352 (2019). FIG. 6A shows spectraof nickel spinels with varying Ni:Al ratio, all calcined at 950° C. andquenched. The spectra of Ni₁ is comparable to those previously reportedfor nickel aluminate spinels, with absorption bands near 380 nm and from600-645 nm ascribed to v₃(³A_(2g)→³T_(1g)(P)) and (³T₁(F)→³T_(1g)(P))electronic transitions of Ni²⁺ within the spinel structure. See M.Gil-Calvo et al., Appl. Catal. B 209, 128 (2017). Lowering the Ni:Alratio below stoichiometric values increases reflectivity across thevisible spectrum, in agreement with previous observations. See S. Zhanget al., Appl. Catal. B 291, 120074 (2021). CIELAB color spacemeasurements were taken to quantify changes in lightness and coloration,as shown in Table V. Lightness, measured as L*, consistently decreasedwith increasing content of photo absorbing nickel cations. a* and b*values were primarily negative for this sample set, indicatinggreen-blue coloration characteristic of nickel aluminates. See B.Serment etal., ACS Omega 5, 18651 (2020). Digital images of thesesamples and others show significant variation in color achieved bymodulating M²⁺:Al³⁺ stoichiometry in nickel, copper, and cobaltaluminates. Non-stoichiometric spinels clearly adopt unique colorsunachieved in the stoichiometric oxide and provide a wider range ofpigment properties.

TABLE V CEILAB chromaticity coordinates for nickel aluminate spinelswith varying Ni²⁺:Al³⁺ ratio. Sample L* a* b* Ni_(0.25) 89.214 −9.24313.294 Ni_(0.5) 85.100 −15.061 −22.658 Ni_(0.75) 81.387 −16.332 −21.595Ni₁ 73.763 −14.778 −13.507

The incorporation of multiple divalent cationic elements was nextevaluated with spectra shown in FIG. 6B. Spectral features are similarfor the Ni₁ and (MgNi)₁ samples, since magnesium does not significantlyabsorb visible light. Co-addition of cobalt lowers reflectance acrossthe visible spectrum, particularly in the 500-700 nm range. Thisabsorption band is ascribed to the ⁴A₂(F)→⁴T₁(P) transition of Co²⁺ ionswithin an aluminate spinel structure. See L. Torkian and M. Daghighi,Adv. Powder Technol. 25, 739 (2014); and R. Chueachot and R. Nakhowong,Mater. Lett. 259, 126904 (2020). Reflectance of (CoCuMgNi)₁ is loweredsubstantially through the addition of copper, which exhibits broadabsorption bands across the visible spectrum due to charge transferbetween oxygen anions and Cu²⁺ occupying octahedral sites. See T.Tangcharoen et al., J. Adv. Ceram. 8, 352 (2019). As shown, HEO spinelscan be highly effective UV-vis absorbers via incorporation of multipletransition metal constituents which absorb light over differentwavelength ranges. Tunable light absorption properties, along with theexcellent thermal stability of aluminate spinels, give these materialsgreat potential for photocatalytic and solar thermal applications. SeeS. R. Atchuta et al., Solar Energy 199, 453 (2020); and T. Tangcharoenet al., J. Mater. Sci.: Mater. Electron. 29, 8995 (2018).

Thermal Stability

Aluminate spinels are commonly used in high temperature environments dueto their excellent thermal stability. However, the desired chemicalproperties of these spinels are application dependent. Chemicalinertness is required of refractory spinel materials, while spinelcatalysts must exhibit chemical interaction with reactant molecules andeven be structurally reducible to form catalytically active metallicphases. See H. Tang et al., Materials (Basel) 14, 3050 (2021); and S.Zhang et al., Appl. Catal. B 291, 120074 (2021). To assess the effect ofvacancies and multiple divalent metals on spinel stability, selectNi-containing samples were evaluated through hydrogen temperatureprogrammed reduction (H₂TPR). FIG. 7 shows the resulting profiles withthe temperature of reduction peaks labeled. All primary reduction peaksoccur at temperatures well over 700° C. and correspond to reduction ofthe nickel aluminate phase. See G. Wang et al., Energy Technol. 7,1800359 (2019). The reduction mechanism was previously identified as aseparation of nickel from the spinel lattice to form metallic nickel andalumina phases. See E. Ustundag et al., Appl. Phys. Lett. 76, 694(2000). Of the spinels evaluated, Ni₁ shows the lowest reduction peaktemperature at 770° C., at which point the spinel structure iscompromised. Inclusion of Co and Mg shifts the reduction peak to highertemperatures. This result agrees with previous observations that CoAl₂O₄and MgAl₂O₄ spinels are less reducible than NiAl₂O₄. See H. Y. Wang andE. Ruckenstein, Catal. Lett. 75, 13 (2001). The reduction peak is alsoshifted to higher temperatures through vacancy inclusion. This shift wasas large as 51° C. between Ni₁ and Ni_(0.5) samples, suggesting thatsub-stoichiometric spinels withstand chemically aggressive environmentsat higher temperatures than stoichiometric counterparts. While aluminaachieves excellent chemical resistance without divalent metalconstituents, alumina phase changes occurring above 800° C.significantly alter the material structure. See S. Lamouri et al., Bol.Soc. Esp. Cerám. V. 56, 47 (2017). Spinel phases, on the other hand, arethermally stable at higher temperatures, but are more chemicallyreactive in conventional stoichiometric form. Since sub-stoichiometricspinels showed no alumina phases, even after 950° C. aging, and showedincreased reduction peak temperatures, cationic vacancies can provide avaluable parameter for optimizing desired thermal and chemicalproperties, which are unachieved in alumina and conventionalstoichiometric spinels. Cationic vacancies yield the added benefit ofreducing processing temperatures (and therefore energy), as well as useof transition metals, many of which are rare yet critical elementsneeded for emerging technologies.

Multi-Cationic Catalysts for Natural Gas Conversion

The multi-cationic aluminate spinels are also useful as catalysts forthe conversion of natural gas constituents. Among other reactions, thisconversion includes the dry reforming of methane (DRM), according to thereaction CH₄+CO₂→2CO+2H₂ DRM utilizes two greenhouse gases; carbondioxide (CO₂) and methane (CH₄) to produce a synthesis gas (syngas)mixture of carbon monoxide (CO) and hydrogen (H₂), which is a veryimportant precursor for the production of a variety of valuablechemicals and liquid fuels.

Therefore, industrial adoption of the DRM reaction would represent asignificant step toward decarbonizing the chemical industry. Dryreforming is usually performed in a temperature range of 650-850° C. Theuse of a catalyst may lead to a reduction of energy needed to drive theendothermic reforming reaction. However, dry reforming is not currentlyperformed industrially due in part to poor catalyst performance.Ni-based catalysts are promising due to their high initial activity andlow cost, but suffer from poor stability due to coke formation and/orsintering of isolated Ni species during the DRM reaction, as shown inFIG. 8A, hindering their commercialization. High temperature oxidationcan effectively combust carbonaceous deposits which form on the catalystsurface during the natural gas conversion reactions and otherwisedeactivate the catalyst. However, this heat treatment also lowers thesurface area of conventional Ni-based catalysts through active particlesintering, lowering performance. As described above, mixtures ofmagnesium, nickel, cobalt, manganese, and other metal cations can beincorporated into the aluminate spinel structure. While nickel is thecatalytically active element, the addition of other metals causesformation of bimetallic nanoparticles during DRM, as shown in FIG. 8B.Under reaction conditions, which include high temperatures and reducingatmospheres, these metals can segregate from the spinel structure toform bimetallic nanoparticles and activate more quickly and withoutpretreatment steps than catalysts containing only nickel, as shown inFIGS. 9A and 9B. In particular, both the stoichiometric andsub-stoichiometric ternary metal aluminate spinels, (CoMgNi)₁ and(CoMgNi)_(0.75), activate rapidly and continuously operate at near 100%CH₄ and CO₂ conversion for in excess of several hours. Thesemulti-cationic catalysts also show improved product selectivity, reducedcoke formation, and excellent stability compared to conventionalmetallic catalysts. SEM and energy dispersive spectroscopy (EDS)analysis, shown in FIGS. 10A-10D, indicate that Co—Ni bimetallicparticles form within both the (CoMgNi)_(0.75) and (CoNi)_(0.67)multi-cationic oxides during DRM reaction. Such Co—Ni bimetallicparticles are known to be selective catalysts. FIG. 11 shows athermogravimetric analysis (TGA) of various spent catalysts after DRMreaction. The TGA scans indicate much less coke formation on themulti-cationic catalysts. Further, the addition of many metals promotesregeneration of the spent catalyst under oxidizing conditions. Inparticular, regenerative treatment causes the segregated metals tore-incorporate into the spinel lattice, which is highly thermallystable. As shown in FIG. 12A, with the Ni-only spinel, NiAl₂O₄, metallicNi particles form due to sintering of isolated Ni species during the DRMreaction. However, during regeneration, these metallic Ni particles formNiO particles under oxidizing conditions. Conversely, as shown in FIG.12B, the multi-cationic catalyst, (CoMgNi)_(0.75)Al₂O₄ forms Co—Nibimetallic particles during the DRM reaction. However, rather thanforming segregated metal oxide particles, the original aluminate spinalstructure is regenerated under oxidizing conditions. This is because theaddition of many metals in these catalysts increases configurationalentropy of the spinel phase, which creates a thermodynamic driving forcefor the consolidation of these elements into a phase pure structure.This prevents sintering and deactivation of the active metal particlesduring regeneration. Therefore, the complex aluminate catalysts of thepresent invention demonstrates excellent stability.

The present invention has been described as multi-cationic aluminatespinels. It will be understood that the above description is merelyillustrative of the applications of the principles of the presentinvention, the scope of which is to be determined by the claims viewedin light of the specification. Other variants and modifications of theinvention will be apparent to those of skill in the art.

We claim:
 1. A multi-cationic aluminate spinel, comprising at leastthree different divalent metal cations in nominally equimolarconcentrations in an aluminate spinel crystal structure.
 2. Themulti-cationic aluminate spinel of claim 1, wherein the at least threedifferent divalent metal cations are selected from the group consistingof alkaline-earth metals and transition metals.
 3. The multi-cationicaluminate spinel of claim 2, wherein the alkaline earth metals comprisemagnesium and calcium and the transition metals comprise cobalt, copper,manganese, nickel, chromium, iron, and zinc.
 3. The multi-cationicaluminate spinel of claim 1, wherein the aluminate spinel crystalstructure is calcined at greater than 800° C.
 4. The multi-cationicaluminate spinel of claim 1, wherein the at least three differentdivalent metal cations comprises a stoichiometric amount of at leastfour different divalent metal cations in nominally equimolarconcentrations.
 5. The multi-cationic aluminate spinel of claim 1,further comprising a plurality of unoccupied cation sites in thealuminate spinel crystal structure and wherein the aluminate spinelcomprises sub-stoichiometric amounts of the at least three differentdivalent metal cations in nominally equimolar concentrations to aconcentration of the plurality of unoccupied cation sites.
 6. Themulti-cationic aluminate spinel of claim 1, wherein the at least threedifferent divalent metal cations comprises nickel, thereby providing aNi-based multi-cationic aluminate spinel catalyst.
 7. The multi-cationicaluminate spinel of claim 6, wherein the Ni-based multi-cationicaluminate spinel catalyst catalyzes the dry reforming of methanereaction.
 8. A method of dry reforming of methane, comprising: providingthe Ni-based multi-cationic aluminate spinel catalyst of claim 6, andexposing a stream comprising methane and carbon dioxide to the Ni-basedmulti-cationic aluminate spinel catalyst at greater than 600° C.,thereby catalytically reacting the methane and carbon dioxide to producea synthesis gas.
 9. The method of claim 8, further comprisingregenerating the exposed Ni-based multi-cationic aluminate spinelcatalyst under oxidizing conditions.